1. Field of the Invention
The present invention relates to a Magnetic Random Access Memory (MRAM) element, and especially to a memory element to be used in a MRAM comprising an array of memory elements.
2. Description of the Related Art
A MRAM element is generally characterized by a magnetic moment vector having an orientation that can be modified by the application of an external action, especially a magnetic field. Specific information is stored in the memory element by orienting the magnetic moment vector along a selected direction and/or orientation. The memory element can keep information for a long time without requiring refreshment of the memory element in the absence of a power supply, which is a significant advantage of this type of memory. As an example, to store binary information, a memory element having a selected-magnetization axis may be provided. The magnetic moment vector is then oriented along a selected-magnetization axis in a first direction to store a first state of the information and in the opposite direction to store a second state of the information.
A conventional method for reading information stored in a MRAM element consists of detecting the resistance differences in the memory element according to the orientation of the magnetic moment vector. A conventional method for writing to a memory element involves write magnetic fields generated by running currents through metal lines external to the memory element. In unconventional fashion, it has also been provided to use the effects caused by a current sent through the actual memory element. Such effects especially are: the magnetic field created by the current, a thermal effect induced by Joule effect, or a transfer effect of the spin angular momentum of the conduction elements towards the magnetization.
FIG. 1 shows the simplified diagram of a conventional magnetic memory. MRAM 10 is formed of first lines of a conductive material forming word lines 12 arranged perpendicularly to second lines of a conductive material forming bit lines 14. Period p of the network of word lines 12 and period p′of the network of bit lines 14 generally are on the order of a few times width w of a metal track and may be different. A memory element 16 is arranged at the intersection of each word line 12 and of each bit line 14. The write process then involves magnetic fields generated by running currents through word lines 12 and bit lines 14.
A realistic diagram of a magnetic memory may be more complex and involve, for example, for reading purposes, a diode or a transistor in series with each memory element with no significant influence upon the present invention.
It is currently attempted to increase the density of magnetic memories, that is, to increase the number of memory elements per surface area unit. A difficulty results due to the fact that, by dipolar interaction, the magnetic moment of a considered memory element is likely to disturb the behavior of adjacent memory elements, the difficulty increases as the memory density increases. A disturbance for example translates as a degradation in the reliability of the process of information writing into the memory elements adjacent to the considered memory element, or as a degradation in the information retention time in the adjacent memory elements.
To avoid such a disadvantage, the use of a memory element, which, in the absence of application of an external magnetic field, has a very small magnetic moment, is preferred. Such a memory element is said to be compensated. A compensated memory element has a negligible influence upon the adjacent memory elements as soon as it is moved away from the memory element by a distance greater than a small multiple of the thickness of the memory element.
FIG. 2 schematically shows an example of a compensated memory element, of the type of the memory element described in U.S. Pat. No. 6,545,906.
Memory element 16 is placed between a word line 12 and a bit line 14, without requiring a specific assumption concerning a possible contact between memory element 16 and word line 12 and/or bit line 14, nor concerning the presence of additional layers between memory element 16 and word line 12 and/or bit line 14. As an example, word line 12 is placed at the top of memory element 16 and bit line 14 is placed at the base of memory element 16 and is directed according to a 90° angle with respect to word line 12.
Magnetic regions 18, 22 are formed by synthetic antiferromagnet structures, also called SAF structures. More specifically, memory element 16 comprises a first magnetic region 18, a barrier layer 20, and a second magnetic region 22, barrier layer 20 being sandwiched between first magnetic region 18 and second magnetic region 22. First magnetic region 18 includes a three-layer structure which includes a separation layer 24 sandwiched between two ferromagnetic layers 26, 28, and which induces an antiferromagnetic coupling between the two ferromagnetic layers 26, 28. Second magnetic region 22 includes a three-layer structure which comprises a separation layer 30 sandwiched between two ferromagnetic layers 32, 34 and which induces an antiferromagnetic coupling between the two ferromagnetic layers 32, 34. Ferromagnetic layers 26, 28 respectively have magnetic moment vectors {right arrow over (μ)}1, {right arrow over (μ)}2 which are maintained antiparallel by coupling through separation layer 24. Similarly, ferromagnetic layers 32,34 respectively have magnetic moment vectors {right arrow over (μ)}3, {right arrow over (μ)}4 which are maintained antiparallel by coupling through separation layer 30. Each ferromagnetic layer may be formed of several ferromagnetic layers coupled by interfacial exchange interactions.
Magnetic region 18 is called a free magnetic region since {right arrow over (μ)}1 and {right arrow over (μ)}2 are free to pivot in the presence of a magnetic field applied to memory element 16. Magnetic region 22 is called a trapped magnetic region since {right arrow over (μ)}3 and {right arrow over (μ)}4 are not free to pivot in the presence of a magnetic field having a moderate amplitude, magnetic moment vector {right arrow over (μ)}4 being set in a selected direction. This result is generally obtained by deposition of layer 34 on an antiferromagnetic film which has no significant influence upon the present invention. Magnetic region 22 can thus be used as a reference magnetic region.
Magnetic regions 26, 28, 32, 34 have substantially parallel selected-magnetization axes along which magnetic moment vectors {right arrow over (μ)}1, {right arrow over (μ)}2, {right arrow over (μ)}3, {right arrow over (μ)}4 orient in selected fashion. The storage of information in a memory element 16 is obtained by orienting {right arrow over (μ)}1 in parallel with or antiparallel to {right arrow over (μ)}3. The reading of the information stored in the memory element is performed by having a current run through the memory element and by detecting the resistance differences in memory element 16 according to the orientation of {right arrow over (μ)}1. Indeed, the resistance depends on the relative orientation of {right arrow over (μ)}1 with respect to {right arrow over (μ)}3.
To avoid magnetic moments of memory element 16 from disturbing the adjacent memory elements, magnetic regions 18, 22 are compensated, that is, the materials and/or the volumes of ferromagnetic regions 26, 28, 32, 34 are selected so that the resulting magnetic moment {right arrow over (μ)}tot of free magnetic region 18, equal to {right arrow over (μ)}1+{right arrow over (μ)}2 and the resulting magnetic moment vector of trapped magnetic region 22, equal to {right arrow over (μ)}3+{right arrow over (μ)}4, are substantially equal to the zero vector. This then enables increasing the density of magnetic memory 10 with respect to a simple memory element, the free magnetic region of which would be formed of a single ferromagnetic layer, as is the case in a conventional memory element.
Further, in a compensated memory element, the minimizing or the optimizing of the interaction, mainly of dipolar origin, between magnetic regions 18 and 22 must be taken into account in the structure details of these regions. For simplification, this point will not be considered in the following description, while having verified that it has no significant influence upon the present invention.
Conventionally, the writing of information into memory element 16 is obtained by the generation of magnetic fields adapted to orient {right arrow over (μ)}1 in a selected direction and way. Since {right arrow over (μ)}1 and {right arrow over (μ)}2 are maintained antiparallel by the antiferromagnetic interaction through layer 24, it is equivalent to control the orientation of {right arrow over (μ)}1 or that of {right arrow over (μ)}2. The write magnetic fields are created by the flowing of currents in word line 12 and bit line 14.
The definition of magnetic fields for the writing of a single memory element 16 is a delicate process. Indeed, it must be avoided for the write magnetic fields resulting from the flowing of currents in a word line 12 and a bit line 14 to cause a writing into other memory elements 16, for example, the memory elements connected to the same word line or to the same bit line, and the memory elements neighboring the addressed memory element. The obtaining of proper write magnetic fields is generally more difficult still when the free magnetic region of the memory element has a compensated or almost compensated structure since the modification of the orientation of the magnetic moment vectors of the memory elements tends to require more intense magnetic fields, in particular to overcome the interaction between the layers of the free magnetic region. Further, when a magnetic field is applied to the memory element, it is difficult to ensure that resulting moment {right arrow over (μ)}tot of free magnetic region 18 maintains during the write process a sufficiently low amplitude to avoid disturbing the adjacent memory elements.
To achieve very high integration densities, two other contradictory phenomena must be controlled. The first phenomena is that when the volume of free magnetic region 18 decreases, the preservation of a sufficient information retention time requires increasing the total magnetic anisotropy of the sample, which, in general, increases the amplitude of the magnetic fields required for the write process. The second phenomenon is due to the fact that the maximum admissible current density in a conductive line is limited by physical phenomena (Joule effect, electromigration). The maximum current that can be injected for a write process and the write magnetic field, which is proportional thereto thus decrease along with the conductive line cross-section as the memory density is increased.
U.S. Pat. No. 6,545,906 describes a specific compensated or almost compensated memory element write process.